Ground contact switch for personal navigation system

ABSTRACT

A ground contact switch system comprises a first object configured to contact a ground surface during a stride, and one or more switches coupled to the first object. An inertial measurement unit can be coupled to the first object. The one or more switches are configured to detect when the first object is at a stationary portion of the stride. The one or more switches can also be configured to send a signal to activate an error correction scheme for the inertial measurement unit.

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 60/973,301, filed on Sep. 18, 2007, thedisclosure of which is incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to copending U.S. application Ser. No.(Docket No. H0016994), and entitled “METHOD OF PERSONAL NAVIGATION USINGSTRIDE VECTORING,” the disclosure of which is incorporated herein byreference.

This application is also related to copending U.S. application Ser. No.(Docket No. H0017062), and entitled “ULTRASONIC MULTILATERATION SYSTEMFOR STRIDE VECTORING,” the disclosure of which is incorporated herein byreference.

BACKGROUND

In a conventional navigation system, an inertial measurement unit (IMU)is used to track the movement of a person, a ground vehicle, or an airvehicle. Additional sensors are typically used in the navigation systemto correct for IMU error growth and drift. The zero velocity update(ZUPT) is a commonly used correction technique for IMU error growth anddrift in a navigation system for a person. This technique requiresknowledge of when one foot of a person is stationary (i.e., at zerovelocity). Currently, this is determined by monitoring accelerometers inthe IMU at all times, to identify a signature indicating that thevelocity is zero.

In using the ZUPT technique, if a foot is known to be stationary, thenthe IMU can be checked to make sure that it, too, is indicating zerovelocity. If it is not, due to error growth or drift, then the IMU canbe corrected. While this technique can reduce IMU error growth and makesthe navigation system more accurate, additional computational time andpower are required, both of which can be extremely limited in anavigation system. Further, while ZUPT algorithms reduce distanceerrors, they cannot effectively bound heading errors. In order to boundheading errors a compass is often used with the ZUPT, however, compassaccuracy still results in limited position performance and is inadequatefor long, precise global positioning system (GPS) denied missions.

Accordingly, there is a need for better techniques to correct for IMUerror growth that provide information on when a foot is stationary,without expending too much computation time or power.

SUMMARY

The present invention relates to a ground contact switch system thatcomprises a first object configured to contact a ground surface during astride, and one or more switches coupled to the first object. Aninertial measurement unit can be coupled to the first object. The one ormore switches are configured to detect when the first object is at astationary portion of the stride. The one or more switches can also beconfigured to send a signal to activate an error correction scheme forthe inertial measurement unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention will become apparent to those skilledin the art from the following description with reference to thedrawings. Understanding that the drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting in scope, the invention will be described with additionalspecificity and detail through the use of the accompanying drawings, inwhich:

FIG. 1 is a graph showing the acceleration signature of a foot in the x(horizontal) and y (vertical) directions over time;

FIG. 2A illustrates one embodiment of a ground contact switch systemthat includes a pneumatic switch in footwear;

FIG. 2B is an enlarged sectional view of the ground contact switchsystem of FIG. 2A;

FIG. 3A illustrates another embodiment of a ground contact switch systemthat includes an electrical switch in footwear;

FIG. 3B is an enlarged sectional view of the ground contact switchsystem of FIG. 3A;

FIG. 4 illustrates another embodiment of a ground contact switch systemthat includes an ultrasonic switch in footwear; and

FIG. 5 illustrates a personal navigation system in use that implements aground contact switch system.

DETAILED DESCRIPTION

In the following detailed description, embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. It is to be understood that other embodiments may be utilizedwithout departing from the scope of the present invention. The followingdetailed description is, therefore, not to be taken as limiting.

The present invention is related to a ground contact switch for use inpersonal navigation systems. The ground contact switch vastly simplifiesdetermination of zero velocity, thus reducing computational requirementsfor inertial measurement unit (IMU) error correction. The ground contactswitch makes ascertaining of ground contact by the foot deterministic,and simplifies high performance personal navigation systems, whichcurrently must determine foot contact by analysis of accelerationsignals.

The ground contact switch can be used in personal navigation systems forboth military and civilian applications. For example, one or more groundcontact switches can be used in personal navigation systems implementedin footwear for soldiers, first responder personnel (e.g., fire, rescue,police), consumer applications, and the like.

The ground contact switch can also be used in navigation systems forrobots that “walk” such as humanoid robots or other multi-leg robots.For example, one or more ground contact switches can be incorporateddirectly into the structure of a robot foot as part of the navigationsystem for the robot.

A ground contact switch system according to one embodiment comprises afirst object configured to contact the ground during a stride, and oneor more switches coupled to the first object. The first object can be afirst item of footwear or a robot foot. The one or more switches areconfigured to detect when the first object is at a stationary portion ofthe stride. The switches can send a signal to activate an errorcorrection scheme for an IMU when used in a personal navigation system.The ground contact switch system can also include a second object thatis paired with the first object, such as a second item of footwear oranother robot foot, and one or more switches coupled to the secondobject. The one or more switches coupled to the second object areconfigured to detect when the second object is at a stationary portionof the stride.

As described previously, the zero velocity update (ZUPT) is a commonlyused correction technique for IMU error growth and drift in a personalnavigation system for a person. This technique requires knowledge ofwhen one foot of a person is stationary (i.e., at zero velocity). Thezero velocity determination is done by monitoring the IMUaccelerometers, or separate accelerometers, at all times, to identify asignature indicating where the velocity is zero. FIG. 1 is a graphshowing such a signature in which the acceleration of a foot in the x(horizontal) and y (vertical) directions over time is measured todetermine the zero velocity points.

Another IMU error correction technique called “stride vectoring” isdescribed in copending U.S. application Ser. No. (Docket No. H0016994),and entitled “METHOD OF PERSONAL NAVIGATION USING STRIDE VECTORING.” Thestride vectoring technique provides heading information that can be usedin non-zero-velocity (motion) IMU error correction. In stride vectoring,while one foot is stationary and the other foot is moving, the positionvector between the two can be measured using techniques such asultrasonic trilateration or multilateration. The IMU in the moving footcan then be updated during the moving portion of the stride (a motionupdate, or MUPT), and not just the stationary part of the stride. Aswith the ZUPT, a MUPT requires knowledge of when one foot is stationary.

The present ground contact switches make two significant improvements inapplying IMU error correction techniques, which simplify thedetermination of zero velocity and thereby reduce computationalrequirements. First, since the foot of a person will have zero velocityonly when on the ground, the ground contact switch activates the errorcorrection algorithm (e.g., ZUPT or MUPT) only during ground contact.This eliminates the need to monitor the accelerometers in the IMU at alltimes, reducing the monitoring to only short time periods. Second,rather than using a ground contact sensor, which provides a continuoussignal that must be processed, the present ground contact switch isused, which is only “on” or “off” (contact or no contact). Thus, verylittle computation is required to make the determination of zerovelocity. The switch either turns on the error correction when theperson's foot contacts the ground, or the switch turns off the errorcorrection when the foot is off the ground. If necessary, the groundcontact switch can trigger a brief check of the accelerometers to verifya zero velocity condition.

The present ground contact switches simplify the identification of azero velocity condition, and trigger the start of a ZUPT or stridevectoring sequence. This requires far less computation time and powerthan the typical zero-velocity determination from full-timeaccelerometer processing.

The ground contact switch can take many forms. For example, the switchcan be a pneumatic switch, a resistive switch, a capacitive switch, orthe like that is coupled to footwear such as a boot or shoe. One or moreswitches can be integrated into the footwear, such as one switch at theheel and another switch near the ball of the foot on the sole. Usingmultiple switches can provide more accuracy in determining thezero-velocity point. Low-cost ground contact switches can be fabricatedusing polymer sheets and conductive materials.

In an alternative embodiment, the ground contact switch is an ultrasonicswitch having a transmitter/receiver (transceiver) embedded in footwear.The transmitted ultrasonic signal reflects off the outer surfaces orinner surfaces of the boot or shoe. The reflection will differ stronglydepending on whether the boot or shoe is in contact with solid ground orair, thereby allowing for switching.

The ground contact switch can be implemented in various embodiments. Inone embodiment, the switch needs to be flat enough to be coupled to aheel, sole, or insole of a shoe or boot. For example, a switch can beembedded in each boot of a pair of boots. One switch triggers a ZUPTcorrection, while the other switch triggers the stride vectoringcorrection. The use of switches rather than sensors dramatically reducesthe computational complexity and power usage of determining the zerovelocity point.

The switch can be built or set to a certain threshold force setting thatrepresents contact with the ground. For example, a force of about 50 lbs(chosen arbitrarily) or more can be used to trigger the switch for aperson weighing 150 lbs. The threshold will vary depending on the weightof the person. A threshold force that is too small would representuncertain ground contact and would not trigger the switch. The switchcan be part of the navigation information algorithm (e.g., Honeywell'sECTOS IIC software) that initiates the error correction (ZUPT or MUPT)algorithm.

Exemplary embodiments of the ground contact switch are describedhereafter with respect to the drawings. Although the various groundcontact switches described and shown are incorporated into footwear fora person, it should be understood that the ground contact switches canalso be incorporated into robotic feet.

FIGS. 2A and 2B illustrate one embodiment of a ground contact switchsystem 200 including a first pneumatic switch 210 coupled to a firstitem of footwear 220, such as a boot or shoe. The footwear 220 has aheel 222 and a sole 224 each with an outer surface that contacts theground during use. The switch 210 is configured to detect when a foot230 inside footwear 220 is at a stationary portion of a step.

As depicted in FIG. 2B, pneumatic switch 210 includes a flexiblereservoir 212 of gas, such as air or nitrogen, which is attached tofootwear 220. The pneumatic switch 210 also includes a pair of switchplates 214, 216 attached to opposite sides of flexible reservoir 212.The switch plates 214, 216 are composed of a conductive material such asaluminum or copper, or layered combinations of metals.

In one embodiment, pneumatic switch 210 can be embedded in heel 222 orsole 224 of footwear 220. In an alternate embodiment, pneumatic switch210 can be mounted on the outer surface of heel 222 or sole 224 offootwear 220

In one embodiment such as shown in FIG. 2A, an inertial measurement unit(IMU) 228 can be coupled to footwear 220 as part of a personalnavigation system. The IMU 228 can be embedded in footwear 220 orattached to an outer surface of footwear 220. The IMU 228 can includemicro-electro-mechanical systems (MEMS) gyroscopes and accelerometersthat are integrated onto a single, six degree-of-freedom (DOF) chip.

The ground contact switch system 200 can further include a second itemof footwear that is paired with the first item of footwear 220, such asa pair of boots or shoes. A switch can be coupled to the second item offootwear similar to switch 210 in the first item of footwear 220. Thesecond switch is configured to detect when a foot inside the second itemof footwear is at a stationary portion of a step. The second item offootwear can have an IMU such as IMU 228 embedded therein, or canexclude the IMU.

During use, pneumatic switch 210 detects that foot 230 inside footwear220 is at a stationary portion of a step when flexible reservoir 212 iscompressed by foot 230 sufficiently to cause switch plates 214, 216 tocontact each other and activate a signal. The signal can initiate anerror correction scheme for a personal navigation system, such as errorcompensation using stride vectoring for IMU 228. The pneumatic switch210 turns on the error correction when foot 230 contacts the ground withsufficient force such as during a stride. The pneumatic switch 210 turnsoff the error correction when foot 230 is off the ground during thestride.

FIGS. 3A and 3B illustrate another embodiment of a ground contact switchsystem 300 including an electrical switch 310 coupled to a first item offootwear 320, such as a boot or shoe. The switch 310 is configured todetect when a foot 330 inside footwear 320 is at a stationary portion ofa step.

As shown in FIG. 3B, electrical switch 310 includes a thin elastic layer312 of a resistive material or a dielectric material, which isinterposed between at least two conductive layers 314 and 316 in astacked configuration. The conductive layers 314 and 316 can be formedof various metallic materials, such as aluminum, gold, platinum, copper,or combinations thereof.

When an elastic resistive material is used for layer 312, the electricalswitch 310 is a resistive switch. Various types of elastic resistivematerials can be used for layer 312. In some resistive materials, theresistance changes due to the thickness change (compression) of layer312 when foot 330 steps on switch 310. In other resistive materials, theresistance changes as embedded conductive particles get closer togetherunder compression.

When an elastic dielectric material is used for layer 312, theelectrical switch 310 is a capacitive switch. Various types of elasticdielectric materials can be used for layer 312, such as various kinds ofrubber (e.g., latex, silicone, neoprene, etc.) that can be formed intothin layers. The capacitance changes due to the thickness change(compression) of layer 312 when foot 330 steps on electrical switch 310.

In one embodiment, electrical switch 310 can be attached to an insole325 of footwear 320. In an alternate embodiment, electrical switch 310can be mounted on the outer (bottom) surface of the heel or sole offootwear 320. In other embodiments, multiple electrical switches (two ormore) can be attached or mounted to the heel or sole of footwear 320.

In one embodiment such as shown in FIG. 3A, an IMU 328 can be coupled tofootwear 320 as part of a personal navigation system. The IMU 328 can beembedded in footwear 320 or attached to an outer surface of footwear220. The IMU 328 can include the same features as discussed above forIMU 228.

The ground contact switch system 300 can further include a second itemof footwear that is paired with the first item of footwear 320, such asa pair of boots or shoes. A switch can be coupled to the second item offootwear similar to switch 310 in the first item of footwear 320. Thesecond switch is configured to detect when a foot inside the second itemof footwear is at a stationary portion of a step. The second item offootwear can have an IMU such as IMU 328 embedded therein, or canexclude the IMU.

During use, electrical switch 310 detects that foot 330 inside footwear320 is at a stationary portion of a step when elastic layer 312 iscompressed by foot 330 sufficiently to change the resistance orcapacitance of electrical switch 310 to a predetermined threshold levelsuch that electrical switch 310 activates a signal. The signal caninitiate an error correction scheme for a personal navigation system,such as error compensation using stride vectoring for IMU 328. Theelectrical switch 310 turns on the error correction when foot 330contacts the ground with sufficient force such as during a stride, andturns off the error correction when foot 330 is off the ground duringthe stride.

FIG. 4 illustrates another embodiment of a ground contact switch system400 that includes an ultrasonic switch 410 coupled to a first item offootwear 420, such as a boot or shoe. The footwear 420 has a heel 422with an outer surface 424 that contacts the ground during use. Theswitch 410 is configured to detect when a foot 430 inside footwear 420is at a stationary portion of a step.

The ultrasonic switch 410 includes an ultrasonic transceiver configuredto transmit and receive an ultrasonic pulse. In one embodiment, theultrasonic transceiver can be configured to outwardly transmit theultrasonic pulse toward outer surface 424 of heel 422. In an alternateembodiment, the ultrasonic transceiver can be configured to inwardlytransmit the ultrasonic pulse toward an insole 426 of footwear 420. Inone embodiment, ultrasonic switch 410 can be embedded in heel 422.

When ultrasonic switch 410 is configured to outwardly transmit, anultrasonic pulse having an incident signal (I) is transmitted towardouter surface 424 of heel 422. A reflected signal portion (R) of signal(I) is reflected back from outer surface 424, and a transmitted signalportion (T) passes through outer surface 424. The intensity of thereflected signal portion will differ depending on whether footwear 420is in contact with solid ground or air. When ultrasonic switch 410 isconfigured to inwardly transmit, an ultrasonic pulse is transmittedtoward insole 426 of footwear 420. As the pressure between foot 430 andfootwear 420 changes during a stride, the reflected signal intensitychanges.

In one embodiment, an IMU 428 can be coupled to footwear 420 as part ofa personal navigation system. The IMU 428 can be embedded in footwear420 or attached to an outer surface of footwear 420. The IMU 428 caninclude the same features as discussed above for IMU 228.

The ground contact switch system 400 can further include a second itemof footwear that is paired with the first item of footwear 420, such asa pair of boots or shoes. A switch can be coupled to the second item offootwear similar to switch 410 in the first item of footwear 420. Thesecond switch is configured to detect when a foot inside the second itemof footwear is at a stationary portion of a step. The second item offootwear can have an IMU such as IMU 428 embedded therein, or canexclude the IMU.

During use, ultrasonic switch 410 detects that foot 430 inside footwear420 is at a stationary portion of a step when an ultrasonic pulseemitted by switch 410 has a reflected signal portion that reaches apredetermined reflection threshold. If the reflected signal portionintensity indicates contact of footwear 420 with the ground by reachingthe predetermined reflection threshold, a switch-on output is producedthat initiates an error correction scheme for a personal navigationsystem, such as error compensation using stride vectoring for IMU 428.If the reflected signal portion intensity indicates contact with airfrom lifting footwear 420 off the ground, a switch-off output isproduced.

The ground contact switches described herein can be implemented as partof a personal navigation system, such as disclosed in copending U.S.application Ser. No. (Docket No. H0017062), and entitled “ULTRASONICMULTILATERATION SYSTEM FOR STRIDE VECTORING.”

FIG. 5 illustrates a personal navigation system 500 during use that canimplement any of the ground contact switch systems described previously.The personal navigation system 500 generally includes a GPS, an IMU, andother optional features. In one embodiment, the IMU can include MEMSgyroscopes and accelerometers that are integrated onto a single chip,which is copackaged with a processor such as an application-specificintegrated circuit (ASIC) to produce the chip-scale ISA. A stridevectoring algorithm, and a ZUPT algorithm can be programmed into theASIC. Optional features for personal navigation system 500 can include athree-dimensional magnetic compass, barometric altimeter, temperaturesensor, motion classification, and a stride vectoring system.

As shown in FIG. 5, a user 512 is wearing a left boot 514 containing anIMU 528, a plurality of non-collinear ultrasonic receivers 530, and aground contact switch 540. A right boot 516 of user 512 has anultrasonic transmitter 550 in operative communication with receivers530, and a ground contact switch (not shown). Although FIG. 5illustrates that user 512 is a soldier, it should be understood thatsystem 500 can be used by other types of personnel such as firstresponders, or consumers.

In an alternative embodiment, a navigation system similar to personalnavigation system 500 can be incorporated into the feet of a robot thatwalks. Such a navigation system for the robot generally includes one ormore ultrasonic transmitters, ultrasonic receivers, one or more groundcontact switches, one or more IMUs, and a GPS.

The present invention may be embodied in other specific forms withoutdeparting from its essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore indicated by theappended claims rather than by the foregoing description. All changesthat come within the meaning and range of equivalency of the claims areto be embraced within their scope.

1. A ground contact switch system, comprising: a first object configuredto contact a ground surface during a stride; one or more switchescoupled to the first object; and an inertial measurement unit coupled tothe first object; wherein the one or more switches is configured todetect when the first object is at a stationary portion of the stride,and send a signal to activate an error correction scheme for theinertial measurement unit.
 2. The system of claim 1, further comprising:a second object that is paired with the first object; and one or moreswitches coupled to the second object; wherein the one or more switchescoupled to the second object is configured to detect when the secondobject is at a stationary portion of the stride.
 3. The system of claim2, further comprising: an inertial measurement unit coupled to thesecond object; wherein the one or more switches coupled to the secondobject is configured to send a signal to activate an error correctionscheme for the inertial measurement unit coupled to the second object.4. The system of claim 2, wherein the first and second objects comprisefootwear.
 5. The system of claim 4, wherein the footwear comprises apair of boots or shoes.
 6. The system of claim 2, wherein the first andsecond objects comprise robot feet.
 7. The system of claim 2, whereinthe switches comprise one or more of a pneumatic switch, an electricalswitch, or an ultrasonic switch.
 8. A ground contact switch system,comprising: a first item of footwear; and one or more switches coupledto the first item of footwear, the switches comprising one or more of apneumatic switch, an electrical switch, or an ultrasonic switch; whereinthe one or more switches is configured to detect when a foot inside thefirst item of footwear is at a stationary portion of a stride.
 9. Thesystem of claim 8, further comprising: an inertial measurement unitcoupled to the first item of footwear; wherein the one or more switchesis configured to send a signal to activate an error correction schemefor the inertial measurement unit.
 10. The system of claim 8, furthercomprising: a second item of footwear that is paired with the first itemof footwear; and one or more switches coupled to the second item offootwear and comprising one or more of a pneumatic switch, an electricalswitch, or an ultrasonic switch; wherein the one or more switchescoupled to the second item of footwear is configured to detect when afoot inside the second item of footwear is at a stationary portion ofthe stride.
 11. The system of claim 10, further comprising: an inertialmeasurement unit coupled to the second item of footwear; wherein the oneor more switches coupled to the second item of footwear is configured tosend a signal to activate an error correction scheme for the inertialmeasurement unit.
 12. The system of claim 8, wherein the pneumaticswitch comprises: a flexible reservoir of gas; and a pair of conductiveswitch plates attached to opposite sides of the flexible reservoir. 13.The system of claim 12, wherein the pneumatic switch is embedded in thefirst item of footwear, or mounted on an outer surface of the first itemof footwear.
 14. The system of claim 12, wherein the pneumatic switchdetects that a foot inside the first item of footwear is at a stationaryportion of a stride when the flexible reservoir is compressed by thefoot sufficiently to cause the switch plates to contact each other andactivate a signal.
 15. The system of claim 8, wherein the electricalswitch comprises: at least two conductive layers in a stackedconfiguration; and an elastic layer of resistive material or dielectricmaterial interposed between the at least two conductive layers.
 16. Thesystem of claim 15, wherein the electrical switch is embedded in thefirst item of footwear, or mounted on an outer surface of a heel or soleof the first item of footwear.
 17. The system of claim 16, wherein theelectrical switch detects that a foot inside the first item of footwearis at a stationary portion of a stride when the elastic layer iscompressed by the foot sufficiently to change the resistance orcapacitance of the electrical switch to a predetermined threshold levelsuch that the electrical switch activates a signal.
 18. The system ofclaim 8, wherein the ultrasonic switch comprises an ultrasonictransceiver configured to transmit and receive an ultrasonic pulse. 19.The system of claim 18, wherein the ultrasonic switch detects that afoot inside the first item of footwear is at a stationary portion of astep when the ultrasonic pulse has a reflected signal portion thatreaches a predetermined reflection threshold.
 20. The system of claim18, wherein the ultrasonic transceiver is configured to transmit theultrasonic pulse toward a bottom outer surface of the first item offootwear, or transmit the ultrasonic pulse toward an insole of the firstitem of footwear.